Closes#58: the Context panel hung off two registrations at once (the
conversation tab and the right-Sidebar guide entry), with no way to keep
just one. A new per-user `defaultPlacement` preference — the first row of
the Plugin configuration card — picks who carries the view: the
conversation tab, the right Sidebar, or both (the default, so existing
deployments see no change).
- shared: `DefaultPlacement` vocabulary (`all` | `tab` | `sidebar`) on the
settings namespace; the Host schema gains a loose `defaultPlacement`
field so a stale persisted value degrades to `all`
- client: `placement.ts` mounts each registration through its own factory
and, on a preference flip, disposes exactly what the new value drops
(the harness's own dynamic-mount idiom); `watchSidebarContextTab`
returns its deferred-inject disposer so the sidebar mount can be taken
down and put back; the settings card leads with the placement row
bowenliang123
a4add07664